Magnetic storage devices are used to store data on a magnetic storage medium through the use of writer and reader elements, which respectively write and read magnetic data on the medium. For example, a disk storage device is generally adapted to work with one or more magnetic recording disks that are coaxially mounted on a spindle motor of the device for high-speed rotation. As the disks rotate, one or more transducers, i.e., read and/or write heads, are moved across the surfaces of the disks by an actuator assembly to read and write digital information on the disks.
Given the general desire to store ever-increasing amounts of digital information, designers and manufacturers of magnetic storage devices are continually attempting to increase the bit density of magnetic storage media. In a magnetic recording disk, this means increasing the areal density, i.e., the number of tracks on a disk and/or the linear density of bits along a given track. New materials, as well as new recording methods, have helped increase the areal density. For example, perpendicular magnetic recording systems have been developed for use in computer hard disk drives. Because magnetic flux is found to magnetize the magnetic recording layer of perpendicular recording media in a vertical orientation, a higher areal density can be achieved compared with longitudinal magnetic recording systems.
Areal densities have also been increased by decreasing the number of magnetic grains in one data bit and by simultaneously decreasing the size of the magnetic grains. However, in the process, media writeability can become an issue due to the need to raise anisotropy of the magnetic media in order to maintain thermal stability of smaller magnetic grains. In addition, many grains (e.g., about 40-60 grains) are still needed to achieve signal-to-noise ratio in a single recording bit due to random placement of the grains. One solution to the above issues has involved the use of bit-patterned media (BPM).
In BPM, the magnetic recording surface is patterned to provide a number of discrete, single-domain magnetic islands (usually one island per bit) separated from each other at pre-determined locations. Because each island location is pre-determined, each recording bit only needs to contain one island, thereby greatly enhancing the areal density. Servo information is often included on the BPM in order to provide positioning information for a servo control system. To this end, during a writing operation on a BPM, a write or recording head can be precisely positioned over a given data array, e.g., data track, to magnetize the bits thereon, where such bits are often referred to as dots. Thus, for example, during the rotation of a magnetic recording disk, the writing process can be carefully synchronized with the dots passing by the head in order to facilitate accurate recording and eventual readback of data to and from the dots.
Further efforts have been made with respect to magnetic storage devices to increase the areal density that BPM now affords. One issue of magnetic recording is that when writer dimension shrinks, the output field becomes weaker and curved. To overcome this, a special arrangement of dots can be designed to maintain a wide writer and reader. For example, on a magnetic recording disk, each concentric track can be provided to hold two or more groupings (e.g., rows) of dots such that a wide writer and reader can still discern individual dots. Accordingly, magnetic storage devices have been configured to operate with a write head and a read head adapted to write and read the two or more groupings of dots during a single pass about the BPM. In reading the dot groupings in such manner, the read head can be controlled to read from the two or more groupings while the corresponding track is moved below the head. Accordingly, the dots of such groupings are generally staggered such that the head can read each of the dots in a single pass above the track.
The write process as described above demands very high placement accuracy of the adjacent dots within the grouping to ensure proper write synchronization. This has proved to be somewhat challenging for master pattern creation with a rotating e-beam system. As such, in a two-row staggered dot arrangement for example, e-beam writing is typically done by initially writing a first row and then subsequently writing a second row. Due to mechanical motion and time-delta between writing of each row, it is difficult to control relative placement of adjacent dots.
FIG. 1 exemplifies the above-described conventional writing technique. A portion of a magnetic recording disk 2 is shown, with four partial rows of dots 4 subdivided in two partial data tracks 6 and 8. In using the above-referenced writing technique, a recording head (not shown) is positioned over one of the data tracks, e.g., data track 6, and writes to a corresponding first row of dots 4, e.g., row 7a, as the disk 2 is rotated below the head. As such, while the disk 2 is generally rotated in direction A, the dots 4 of the first row are sequentially written to in direction B (the direction of the arrows between the dots 4). Following the first row being written, a second row, e.g., row 7b, of such data track is similarly written to as the disk 2 is rotated. Subsequently, the recording head can be positioned over the adjacent data track, e.g., data track 8, so that the corresponding rows, 9a and 9b, of such track can be likewise written to.
As described above, writing to a single grouping of dots in a data array presents the challenge of accurately synchronizing the writing process. However, further difficulty is encountered when two or more groupings of dots of a data array are written to so that the groupings can be subsequently read in a single pass by a recording head. For example, with respect to two adjacent rows of dots, this further challenge involves synchronizing the second written row so that each dot therein is accurately written to with respect to the first written row of dots. Such accurate placement of data with respect to the second row of dots, among other things, minimizes errors in subsequent readback of the dot groupings. As is known, the placement accuracy of the adjacent row has been generally dependent on precision phase-lock of pattern clock to spindle encoder. However, errors have been found to occur from deficiencies in maintaining this position accuracy, as described below.
When recording data on magnetic recording disks, magnetic storage devices have typically been provided with an encoder at a bottom end of the spindle (the end opposite to that of the spindle on which the turntable is mounted) so as to provide a precise motor control. In other words, the encoder is mounted in such position such that there is substantially no eccentricity with respect to the axis of rotation of the spindle. This allows for a very precise control of the motor based on the encoder signals. However, such positioning of the encoder is problematic when used to provide a clock, position or velocity source for the format signal generation process during recording. This is due to mechanical vibrations, however slight, which occur in the rotating portions of the recording system. In particular, the vibrations at the top of the spindle (where the turntable is located) and at the bottom of the spindle (where the encoder is located) are not synchronized. Because the distances employed in data tracks are extremely small (generally in the range of nanometers), even minute disturbances can create phase error problems, e.g., between paired dot groupings. It is desirable to minimize these types of errors.